General Features of development in animals

General Features of development in animals

A multi-cellular begins life as a fertilized egg or zygote. The further development depends on the following processes.

Growth

• Growth occurs by the following three basic mechanisms:
  1. An increase in the number of cells — In the early stages of development increase in size by cell proliferation is a major growth mechanism.
  2. An increase in the size of the cells – A good example of growth that occurs by an increase in cell size is the growth of muscles brought about by an increase in exercise. Muscle cells are unable to undergo mitosis and therefore cannot grow by cell proliferation. Thus well – exercised muscle grow because the individual cells increase in size.
  3. An increase in the amount of extracellular material – the best example for this is found in the various connective tissues, which are primarily composed of extracellular material.

Differentiation

• Differentiation occurs in two ways-
  1. Morphogenesis: This refers to a change in the shape and organization of the body and its parts.The more morphogenetic processes in molding of the body and its organs into form and pattern are as follows
     • Cell migration
     • Cell aggregation forming
       • Masses
       • Cords
       • Sheets
     • Localized growth resulting in
       • enlargements of various kinds 
       • Constrictions
     • Fusion and splitting
     • Folding including circumscribed folds which produce
       • Anterior pocketing or evaginations
       •  Bending where folding is due to unequal growth.
     • The differential growth resulting in enlargements, bending and folding of all sorts is the chief process utilized by the embryo in molding its general form and producing new organs.
     • The term “primordium” or “anlage” is applied to the celluar beginnings of a future tissue, organ or part before it assumes the characteristic differentiation and grow features.
  2. Histogenesis
     • This applies to a change in the substance and structure of the cells and therefore various tissues are created.
     •  The total process by which cells differentiate into distinctive kinds and assume specific tissue characters is known as histogenesis.
     • At this early period of differentiation in form and structure they are often designated by the suffix – blast. Eg.Neuroblast will complete its differentiation into a nerve cell and a Myoblast into a muscle cell.
     • Further the histogenic differentiation is the history of the originally single germ layer. Eg. The cells from the ectodermal layer proliferate and gradually change their form and character as they produce
       • A layers of epidermis
       • More specialized epidermal products are the hairs, nails, lens of the eye and enamel of the teeth.
       • Glandular derivative of the ectoderm vary from salivary gland, sweat gland, mammary gland and anterior lobe of pituitary gland.
       • Other local specializations produce the sensory epithelium of the organs of smell, hearing and vision.
       • Part of the ectoderm becomes thickened neural plate from which both nerve cells and supporting elements arise.
• Morphogenesis and histogenesis are the processes resulting in the organogenesis, which resolve the early embryo into complete organs.

Integration

Although the new organs and organ systems possess structural coherence and unity, they need to be reintegrated into co-operative working mechanisms. This control is given by the nervous system and endocrine glands, which constitute the primary mechanism of physiological control and co-ordination. The supplying of organs with adequate nervous, vascular and hormonal influences will make the development to pass from a prefunctional period to a functional period.